Statement of Work 

Exploratory investigation of statistical confidence on the upper quantiles and “peak” 

concentrations of currently available Atrazine monitoring data 

Syngenta contact: Wenlin Chen, Email: Wenlin.chen@syngenta.com, Tel.: 336-632-2015 

Peter Hertl, Email: peter.hertl@syngenta.com, Tel.: 336-632-6483 

RTI contact: Roy W. Whitmore, Email: rww@rti.org, Tel.: 919-541-5809 

Objectives 

The objectives of this exploratory work are to determine 

a) What sample size is sufficient, under simple random sampling, to have a 

specified level of confidence, say 95%, that the maximum value observed in 

the sample exceeds a specified upper population quantile, say the 99th

percentile? 

b) What are the upper quantile values (95th, 99th and 99.9th) and their two-sided 

confidence limits based on the available data: 

a. Safe Drinking Water Act data 

b. Syngenta Voluntary Monitoring Program data and the Atrazine 

Monitoring Program data 

c. The USDA PDP dataset 

d. Combined data sets, where feasible 

c) How do the upper quantile values and confidence limits vary across various 

temporal analysis domains (to be defined by RTI and Syngenta). 

Note: RTI will compute statistical analysis weights for each analysis dataset to account for 

temporal sampling. The weight for a sample will be based on the time period from that 

sample to the next sample from the same CWS. Extreme weights will be reviewed and may 

be truncated. 

Data 

Data will be provided by Syngenta in Excel format (see attached example spreadsheet 

headers at end of this document). For information, a brief description of the data sets is 

provided below. 

1. SDWA data: These data were generated by state agencies under the Safe Drinking 

Water Act and cover the time frame beginning in 1993 to the most recent 2008 with 

available data. The SDWA requires monitoring of finished drinking water at all CWS 

in the United States. Only surface water SDWA data will be used in this analysis. 

For atrazine, sampling frequencies are predominantly quarterly or annually for most 

CWS drawing raw water from surface water sources. 

2. Syngenta Voluntary Monitoring Program (VMP): This program was initiated in late 

1993 and full year data were available from 1994 to 2002. The program monitored 

both raw and finished water. Sampling frequency typically exceeded 20 samples per 

year and collected up to a maximum 30 raw and finished water samples per year. A 
total of 120+ CWS participated in the program. The CWS were invited to join the 

program when the SDWA annual average concentration approached 3 ppb. 

3. Atrazine Monitoring Program (AMP): This data set was initiated in 2003 and 

continues to date. CWS for this program were selected based on past annual average 

concentrations in SDWA monitoring. All sites exceeding 1.6 ppb atrazine as an 

annual average at least once since 1997 participate in the program for a minimum of 5 

consecutive years. Up to 151 CWS have participated in the program since 2003 . A 

high sampling frequency regime (32-35 samples/year for raw and 32-35 for finished 

water) was adopted, with typically four sampling events per month (approximately 

weekly) in April, May, June, and July, and twice per month (biweekly) for the 

remainder of the year. 

4. USDA PDP data: EPA will help to obtain monitoring data from finished water 

samples of community water systems.

Scope of work 

RTI will perform the analyses necessary to accomplish the objectives set out above on the 

timeline specified in the table below. 
Schedule of deliverables 

Deliverable Due Date 

RTI receives all datasets from Syngenta 12/23/2009 

RTI reviews the datasets and prepares them for analysis, including 

temporal analysis weights 

01/15/2010 

RTI and Syngenta define temporal analysis domains 01/22/2010 

RTI delivers preliminary estimates to Syngenta 01/29/2010 

RTI receives Syngenta’s comments regarding the preliminary estimates 02/05/2010 

Draft report delivered to Syngenta 02/12/2010 

If requested, RTI can participate in a meeting with EPA and Syngenta 

to discuss the results 

No later than 

02/26/2010 

Final report delivered to Syngenta 10 working days 

after EPA meeting 

Respond to comments from EPA and Syngenta From the EPA 

meeting through 

06/30/2010 
Example spreadsheet headers of the monitoring data sets. 

PWSID CWS Name State 

Source 

Type 

Water 

Movement Program Year 

Water 

Type Comment LOQ Method 

Sampling Date 

Atrazine (ppb) Wat

KS2020508 Altoona KS S Flowing AMP 2004 F <MDL 0.10 IA 20-Dec-2004 0.05 

VerdRive

KS2020508 Altoona KS S Flowing AMP 2005 F <MDL 0.10 IA 4-Jan-2005 0.05 

VerdRive

KS2020508 Altoona KS S Flowing AMP 2005 F <MDL 0.10 IA 19-Jan-2005 0.05 

VerdRive